It is well-known in the art to use electric motors to provide power assisted steering in motor vehicles. Typically, these power steering systems include a conventional multi-axis gear reduction box between the motor and the steering shaft to raise the torque applied by the motor to the shaft.
When these systems are functioning properly, the motor, in response to the signals from a control circuit, operates to assist the driver of the vehicle by turning the steering shaft via the gear reduction box in the same direction and at the same speed at which the driver turns the steering wheel while boosting his input torque by a factor that can be chosen during design.
However, power assisted steering systems occasionally malfunction. As a result, the motor may either cease to operate entirely or it may operate eratically or continuously, because of malfunctions of the control circuit. When the motor fails to operate entirely, the driver must steer the vehicle without power assistance and, in addition, part of the effort required to turn the steering wheel is absorbed by the motor inertia since, even though it is idle, it remains coupled to the steering shaft.
The second type of failure can be more troublesome. Since the power assist motor may be more powerful than the driver, when the motor fails to respond properly to the driver's commands, the possibility exists that the motor will control the operation of the steered wheels regardless of the driver's actions. In these instances, the driver may lose steering control over the vehicle.